This invention relates to measurement tools useful in the quality control of hydraulic tube fittings, and in particular to an X-control gage used in measuring the X-dimension of hydraulic tube fittings.
On Oct. 4, 1951 the original design drawings for flareless hydraulic tube fittings were issued by the United States Department of Defense under part numbers MS33514 and MS33515. The MS33514 drawing covers the standard flareless fitting end design whereas the MS33515 drawing covers the same fitting end design but has a central flange for use in mounting on a bulkhead, with one end of the fitting meant to protrude through a wall or other surface.
Both types of fittings are used to connect lengths of tubing together in aircraft to convey hydraulic fluids, fuels, and the like. The fittings are also used in land vehicles and other ground support equipment, both military and commercial. In addition, prime contractors to the government design their own special parts utilizing the MS33514 and MS33515 end design.
The government procurement specification used in buying fittings using the MS33514 and the MS33515 end design is MIL-F-18280. This procurement specification refers to MIL-STD-1655, which describes and classifies the various defects that may occur in manufacturing these parts and the effect that the defects might have on usability and safety. The defects are classified in order of importance as "major," "minor A," and "minor B." Major defects have a definite effect on safety and usability. Minor A defects could have an effect and minor B defects should have no effect.
Researching MS33514 and MS33515 in MIL-STD-1655 shows that both of these drawings have only one listed major defect, namely the "X" dimension. This "X" dimension can be defined as that distance from the theoretical sealing area on the 12-degree taper to the tube stop shoulder at the bottom of the port. If the "X" dimension is not within the appropriate tolerance, the parts can leak, causing a loss of hydraulic pressure with a subsequent loss of control functions depending upon that pressure. Obviously, if the "X" dimension is not correct the parts cannot and should not be accepted or used.
Historically, since 1951 when the drawings were originally issued, the only way to determine whether or not the "X" dimension was within tolerance was to use a so-called "X-control gage." These gages were manufactured by several source but they all utilized the same specific design. The design included a spring-loaded plunger inside a body with one end tapered and the other having a 0.010 step ground into the other end of the body.
Testing for part acceptance of a hydraulic fitting with these prior-art gages consisted of inserting the spring-loaded plunger into the port and resting it on the tube stop with the tapered end in contact with the internal taper in the fitting. Theoretically, if the opposite end of the plunger occupied a position anywhere between the upper and lower boundaries of the step created by the 0.010 slot, a part under test could be accepted.
This type of gage has defects which render it not only inaccurate but physically inappropriate for its intended function. The "B"-diameter shown on MS33514 and MS33515 is the theoretical diameter where the sealing function takes place during connection between a hose and the end of he fitting. This diameter must be accurate to within 0.0002 (plus or minus 0.0001). The way the taper on existing gages is formed, it is virtually impossible to determine where the "B"-diameter lies along the taper.
A full degree of tolerance (plus or minus 0.5 degree) on the sealing taper is allowed by MS33514 and MS33515. The tolerance exists to accommodate the tool grinders who manufacture the porting tools. It also allows for tool wear during production.
The only possibility for even moderately accurate use of an existing X-control gage is if the taper on the gage matches the taper on the tool exactly. If the tool taper is ground at the low-taper limit of 11 degrees 30 minutes and the gage taper is ground to 12 degrees 30 minutes, the gage will not work at all. If the tool taper is ground to the high-taper limit of 12 degrees 30 minutes and the gage taper is at the low-taper limit of 11 degrees 30 minutes, the "B"-diameter will never make contact on the taper.
The use of the 0.010-inch ground step at the back end of the gage only allows a visual "pass-fail" indication. Conventional X-control gages provide no way to determine actual numerical values of the quantity being measured. In the best case, using a properly calibrated, properly produced gage, one can effectively reject blatantly defective parts but in the process will also reject a quantity of acceptable parts manufactured to the outer limits. In the worst case, a gage which is improperly produced and calibrated will allow the acceptance of all defective parts. Without the provision of specific numerical readings it is impossible to uncover trends or differences in defects.
The manufacturers of conventional X-control gages have historically failed to provide specific calibration verification results to purchasers. This has occurred systematically due to the inherent flaws in the manufacture and application of these gages. The gages have been sold strictly through the use of "certificates of conformance" which state only that the gage meets the requirements of MS33514 and MS33515. Actual calibration results are never supplied because the results would show that the gage in fact does not meet the requirements.
Despite the defects and inadequacies of conventional X-control gages, they have been used without any substantial change in design for over 30 years. There has been a long-felt need for a gage that will yield actual numerical values that corroborate the accuracy of the "X" dimension and that can be calibrated easily and specifically for accuracy.